Method for improving transistor performance through reducing the salicide interface resistance

ABSTRACT

An embodiment of the invention reduces the external resistance of a transistor by utilizing a silicon germanium alloy for the source and drain regions and a nickel silicon germanium self-aligned silicide (i.e., salicide) layer to form the contact surface of the source and drain regions. The interface of the silicon germanium and the nickel silicon germanium silicide has a lower specific contact resistivity based on a decreased metal-semiconductor work function between the silicon germanium and the silicide and the increased carrier mobility in silicon germanium versus silicon. The silicon germanium may be doped to further tune its electrical properties. A reduction of the external resistance of a transistor equates to increased transistor performance both in switching speed and power consumption.

RELATED CASES

This application is a divisional application of application Ser. No.12/655,341, filed on Dec. 29, 2009 which is a continuation applicationof application Ser. No. 11/899,881 filed on Sep. 7, 2007 which is adivisional application of application Ser. No. 11/171,097 filed on Jun.29, 2005 now U.S. Pat. No. 7,274,055 which is a divisional applicationof application Ser. No. 10/731,269 filed Dec. 8, 2003 now U.S. Pat. No.6,949,482.

FIELD

Embodiments of the invention relate to high speed semiconductortransistors, and more specifically to increasing transistor performanceby utilizing silicon germanium and improved methods of applicationthereof.

BACKGROUND

Silicon complementary metal oxide semiconductor (“CMOS”) technology is adominant microelectronic technology. CMOS offers high reliability, highlevels of integration, low power dissipation, and is verycost-effective. For lower frequency applications CMOS will most likelyremain the dominant technology. However, electron and hole mobility insilicon limits the extent to which CMOS devices can be utilized forhigher speed applications such as radars and mobile communicationdevices that require high transistor switching rates.

One historical solution has been to use semiconductor compounds insteadof elemental semiconductors such as Group IV silicon and germanium.These compounds can be binary, tertiary, and quanternary combinations ofGroup II (Zn and Cd), Group III (B, Al, Ga, and In), Group IV (C, Si,and Ge), Group V (P, As, and Sb) and Group VI (S, Se, and Te) elements.Common III-V semiconductors include Gallium Arsenide (GaAs), GalliumPhosphide (GaP), and Indium Phosphide (InP). Gallium Arsenide, inparticular, has widespread use as a source of and sensor to nearinfrared light given its 1.43 electron volt (“eV”) band gap and as theprimary semiconductor for high speed electronic devices. Despite thespeed improvements over silicon CMOS devices, GaAs is for mostapplications cost prohibitive. One estimate indicates that per squaremillimeter in 1995 dollars, silicon CMOS has a $0.01 cost while GaAsepitaxy has a $2.00 cost.

A newer approach, and one that offers the speed benefits of GaAs andimproved cost-effectiveness of silicon CMOS, employs silicon germanium(strained or unstrained, usually denoted more precisely bySi_(1-x)Ge_(x) or simply as SiGe) and/or strained silicon. Germanium hasa 4.2% larger lattice constant (e.g., atomic spacing) than silicon.Silicon germanium also has a larger lattice constant, the extent ofwhich depends on the percentage composition of germanium. When siliconis grown on silicon germanium, under proper conditions the siliconlattice stretches to match that of the silicon germanium at thesilicon/silicon germanium interface. When silicon germanium is grown onsilicon, under proper conditions the silicon germanium lattice getscompressed. For each method, there is critical thickness of the grownlayer (be it silicon or silicon germanium) past which the grown layerrelaxes as lattice defects propagate.

There are two reasons why strained silicon and silicon germanium offerimproved speed characteristics for transistors comprised thereof.Compared to elemental silicon, germanium has a lower electron effectivemass and lower hole effective mass (leading to higher electron mobilityand higher hole mobility). Silicon germanium compounds benefit from theincreased mobilities of the constituent germanium. Further, the inducedstrain in silicon or silicon germanium (tension and compressionrespectively) creates an anisotropic structure that alters theconduction and valence bands of the materials. When combined with othersemiconductor layers (e.g., heterolayers) with different band gaps,conduction band and valence band discontinuities can be designed tocreate quantum wells or built-in electric fields to accelerate carriersacross the heterolayers.

Silicon germanium deposition can be incorporated into CMOS process flowsrelatively easily. For example, the only major increase in cost is theaddition of a silicon germanium epitaxy step. Given the ease ofintegration and the band gap engineering possible with silicon germanium(e.g., bulk silicon, bulk silicon germanium, and strained variants ofeach) the possibility of manufacturing an entire system on one siliconor silicon on insulator (“SOI”) substrate is real. Integrated systemscould include fiber optic connections, waveguides, optical detectors,CMOS, heterojunction bipolar transistors, and quantum devices all on thesame chip.

Simply using strained silicon and/or silicon germanium does not renderimmediately superior devices. As with all paradigm shifts, incorporatingstrained silicon and silicon germanium into current semiconductorprocessing flows creates a new set of problems to solve.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a: illustration of a substrate cross section following theformation of the gate and nitride spacers

FIG. 1 b: illustration of a substrate cross section following thedeposition of a dielectric film over the entire substrate surface

FIG. 1 c: illustration of a substrate cross section following thepatterning and etching of the dielectric film to expose the source anddrain regions

FIG. 2: illustration of a substrate cross section following the sourceand drain region undercut etch

FIG. 3: illustration of a substrate cross section following thedeposition of silicon germanium in the undercut etched source and drainregions

FIG. 4 a: illustration of a substrate cross section following thedeposition of a refractory metal

FIG. 4 b: illustration of a substrate cross section following aformation anneal to form a silicide contact layer on the surface of thesilicon germanium source drain regions and gate region

FIG. 4 c: illustration of a substrate cross section following theremoval of unreacted refractory metal

FIG. 5: illustration of the band structure of p-type silicon versussilicon germanium

FIG. 6: illustration of the band structure of p-type silicon in contactwith a metal

FIG. 7: illustration of the band structure of silicon germanium incontact with a metal

FIG. 8: illustration of a substrate cross section indicating theexternal resistance (R_(ext)) of the metal oxide semiconductortransistor

DETAILED DESCRIPTION

Embodiments of a method for improving transistor performance will bedescribed. Reference will now be made in detail to a description ofthese embodiments as illustrated in the drawings. While the embodimentswill be described in connection with these drawings, there is no intentto limit them to drawings disclosed therein. On the contrary, the intentis to cover all alternatives, modifications, and equivalents within thespirit and scope of the described embodiments as defined by theaccompanying claims.

An embodiment of the invention reduces the external resistance of atransistor by utilizing a silicon germanium alloy for the source anddrain regions and a nickel silicon germanium self-aligned silicide(i.e., salicide) layer to form the contact surface of the source anddrain regions. The interface of the silicon germanium and the nickelsilicon germanium silicide has a lower specific contact resistivitybased on a decreased metal-semiconductor work function between thesilicon germanium and the silicide and an increased carrier mobility insilicon germanium versus silicon. The silicon germanium may be doped tofurther tune its electrical properties. A reduction of the externalresistance of a transistor equates to increased transistor performanceboth in switching speed and power consumption.

The first transistor created in 1947 was germanium. However, given itsnarrow band gap of 0.67 electron volts (versus 1.11 electron volts forsilicon), reverse-biased germanium p-n junctions exhibit large leakagecurrents. This limited the operating temperature of germanium to below100° C. In addition, it is difficult to manufacture a passivation layeras required by semiconductor processing techniques. Germanium oxide, forexample, is water soluble and dissociates at 80° C. These qualities,coupled with electronics-grade germanium demanding an order of magnitudehigher cost versus silicon, have virtually eliminated elementalgermanium from modern semiconductor technology.

There are, however, benefits to using germanium versus silicon. Forexample, at room temperature, germanium has a electron mobility of 3600cm²/V-s compared to 1350 cm²/V-s for silicon. Even more striking isgermanium's hole mobility of 1800 cm²/V-s versus 480 cm²/V-s forsilicon. As germanium has an intrinsic carrier concentration of 2.5*10¹³cm-3 and silicon has 1.5*10¹⁰ cm-3 at 300K, given that conductivity isproportional to the product of the sum of the mobilities and theintrinsic carrier concentration, germanium has a significantly higherconductivity. As will be more further described below, the performanceof a transistor is related to the external resistance. Given thatresistivity is the inverse of conductivity, utilizing a more highlyconductive material increases the performance of a transistor. Alloyingsilicon and germanium provides the ability to tune the material toutilize the benefits of each constituent semiconductor. As will bedescribed with reference to an embodiment of the invention, asemiconductor alloy of silicon and germanium offers promisingimprovements in certain semiconductor applications.

FIG. 1 a illustrates a substrate cross section following variousprocessing steps to begin creation of a metal oxide semiconductor(“MOS”) transistor. One skilled in the art will recognize whatprocessing steps have occurred, the explanation of which will beomitted. In an embodiment of the invention, the transistor is a p-typeMOS or PMOS. Substrate 100 is silicon. An isolation barrier 101 servesas a channel stop to prevent parasitic effects between closely arrayedtransistors in an integrated circuit application. The isolation barrier101 may be, for example, a shallow trench isolation (“STI”) regionformed by etching a trench in the substrate 100 and filling the trenchwith a deposited oxide isolation material. A gate 102 has been formedand patterned atop an insulator 104, the composition of the gate 102being, for example, polycrystalline silicon. The polycrystalline siliconof gate 102 may be further pre-doped. On each side of the gate 102 thereis a sidewall spacer 103 usually formed of silicon nitride. Eachsidewall spacer 103 serves as a hard mask for subsequent self-alignedprocessing steps. One skilled in the art will understand that, forexample, the sidewall spacer 103 is a hard mask for high dose implantsin a lightly doped drain transistor design or other designs that wouldbenefit from the sidewall spacing as is well known in the art.

Through the processing steps illustrated by FIG. 1 a, the process asdescribed is a standard CMOS process flow as is well known in the art.The next processing step after FIG. 1 a for a CMOS flow would be tocreate the source and drain regions of the MOS transistor by doping thesource and drain regions by ion implant. However, at this point theprocess of an embodiment of the invention departs from a standard CMOSprocess flow. In an embodiment of the invention the silicon germanium isused for the PMOS devices only. Instead of a source and drain regionimplant (i.e., the next step in a standard CMOS process flow), theexposed surface of the wafer is covered with a dielectric layer of, forexample, SiO₂ or Si₃N₄ as illustrated by dielectric layer 104 in FIG. 1b. The dielectric layer is patterned using any known photolithographicor related patterning technique to expose the source and drain regionsof the intended PMOS devices as illustrated by FIG. 1 c, leaving theintended NMOS devices completely covered. An SF₆-based plasma etch thenselectively removes the exposed silicon substrate 100 material in thesource and drain regions of the PMOS device. The etch is selective inthat it removes the bulk silicon substrate 100 material at a much higherrate than the SiO₂ or Si₃N₄ dielectric layer 105 and sidewall spacer 103material serving as an etch mask. The etched source and drain regionsare then selectively filled with silicon germanium (in an embodiment, insitu doped silicon germanium). The dielectric layer 105 serving as amask for the source and drain region etch and silicon germaniumdeposition is then removed using, for example, an HF-based wet etch. Asilicide layer is formed to provide contact to the source, drain, andgate regions of the PMOS and NMOS. The wafer can then undergo theremaining CMOS process steps to create devices that benefit from anembodiment of the invention. More specific processing techniques of anembodiment of the invention will be described in turn.

FIG. 2 illustrates the substrate cross section of FIG. 1 c following anundercut etch 201 to remove substrate 100 silicon from the source anddrain regions of the PMOS devices as introduced above. The profile ofthe etch is such that substrate 100 material has been removed frombeneath sidewall spacers 103. In an embodiment, the undercut etch 201extends beneath the gate 102. The undercut aspect of the undercut etch201 has a substantial impact on the performance benefit created by anembodiment of the invention.

Specifically, as shown in FIG. 2, the undercut etch 201 is formed insubstrate 100 along laterally opposite sidewalls of gate 102. In anembodiment an isotropic etch process is used to form the undercut etch201. An isotropic etch not only etches vertically into the substrate butalso etches horizontally (laterally) beneath each sidewall spacer 103and in an embodiment beneath gate 102. Such a lateral undercut etch canbe produced by using an isotropic dry etch process in, for example, aparallel plate RF plasma etching system using a gas mix chemistrycomprising SF₆ and helium and process conditions that favor isotropy.Such conditions include high pressure and low RF power density. In anembodiment, a process parameters include a pressure of approximately 900mT, a gap of 1.1 cm, an RF power of 100 W, a helium flow of 150 sccm,and a SF₆ flow of 100 sccm is used. RF power may be varied in a range,for example, of 50 W to 200 W, and the process pressure may be variedbut should be greater than approximately 500 mT. In an embodiment theundercut etch 201 has a maximum vertical depth of between 100 and 1500angstroms below the surface of substrate 100 and extends between 25 to200 angstrom horizontally or laterally beneath the gate 102 edge at thesubstrate 100/insulator 104 interface. It is to be appreciated thatalternative process conditions and etch chemistries (e.g., a wet etch)can be utilized to generate other undercut etch 201 geometry profiles isdesired.

Not only does such an etching process produce lateral undercuts beneatheach sidewall spacer 103 and in an embodiment beneath the gate 102 butthe etch chemistry is also highly selective to the insulator 104 oxideand to the sidewall spacer 103 nitride material. In this way theundercut etch does not attack the insulator and sidewall spacer 103material and the geometry of each is preserved.

The etch chemistry used to form the undercut etch 201 is furtherslightly oxidizing. Utilizing an oxidizing etchant causes a portion ofthe insulator 104 layer exposed during the undercut etch process tobecome thicker than the unexposed portion of the insulator 104 layer. Byincreasing the thickness of the insulator 104 layer at the edge of thegate 102, the gate edge leakage at the tip overlap region of the deviceis reduced. A thicker insulator 104 layer at the gate 102 edge helps toincrease the breakdown voltage of the device.

A further advantage of the undercut etch 201 process is that the etchrate reduced to between 5 to 30 angstroms per second, causing the etchof the silicon substrate to concave inwards. With this geometry, a largeL_(MET) (metallurgical channel length or physical channel length) isachieved during the off state of the MOS transistor (low I_(off)) whilea smaller L_(MET) is realized during the on state of the MOS transistorwhen the channel is formed. A smaller L_(MET) during the on statedirectly translates to a smaller channel resistance and accordinglyhigher Ion.

FIG. 3 illustrates the substrate cross section of FIG. 2 following thedeposition of silicon germanium 301 in the undercut etch 201 source anddrain regions. The silicon germanium, as noted, can be represented bySi_(1-x)Ge_(x). The domain of x is [0,1] ranging from pure silicon topure germanium, and can be adjusted to tune the conductivity and bandgap to the requirements of a particular device. In an embodiment, x isapproximately between 0.05 and 0.5 (e.g., approximately between 5% and50% atomically germanium in the silicon germanium alloy). In anotherembodiment, x is approximately between 0.1 and 0.4 (e.g., approximatelybetween 10% and 40% atomically germanium in the silicon germaniumalloy). In yet another embodiment x is approximately between 0.15 and0.3 (e.g., approximately between 15% and 30% atomically germanium in thesilicon germanium alloy). The band gap energy associated with thesilicon germanium 301 alloy can be approximated by the followingequations:E _(g)(x)=(1.155−0.43x+0.0206x ²)eV for 0<x<0.85   (1)E _(g)(x)=(2.010−1.27x)eV for 0.85<x<1   (2)

In an embodiment, therefore, according to equation (1) the band gapenergy of the silicon germanium 301 is approximately between 1.13 eV and095 eV for 5% atomically germanium and 50% atomically germaniumrespectively. In another embodiment, the band gap energy of the silicongermanium 301 is approximately between 1.11 eV and 0.99 eV for 10%atomically germanium and 40% atomically germanium respectively. In yetanother embodiment the band gap energy of the silicon germanium 301 isapproximately between 1.09 eV and 1.03 eV for 15% atomically germaniumand 30% atomically germanium respectively.

The silicon germanium 301 is deposited by selective epitaxial depositionin that the silicon germanium is deposited only on the bulk siliconsubstrate surface exposed by the undercut etch 201 and uncovered by thedielectric layer 105. The silicon germanium 301 crystal does not grow onthe SiO₂ or Si₃N₄ dielectric layer. In an embodiment, the depositiontechnique is reduced pressure chemical vapor deposition (“CVD”)epitaxial deposition. In other embodiments, the deposition techniqueincludes atmospheric CVD epitaxy and ultra high vacuum CVD epitaxy. Eachdeposition technique is a specific form of vapor phase epitaxy as thedeposited silicon germanium 301 is single crystal.

As noted, in an embodiment, the silicon germanium deposition method isCVD epitaxy. Environmentally, the epitaxy occurs between 600° C. and800° C. at a pressure between 10 and 760 torr. Either H₂ or He can beused as a carrier gas. The silicon source precursor gas can be SiH₂Cl,SiH₄, or Si₂H₆. In an embodiment, GeH₄ is the germanium source precursorgas. HCl or Cl₂ may be added as an etching agent to increase thematerial selectivity of the deposition. In an embodiment, the resultingsilicon germanium 301 deposited in the undercut etch 201 source anddrain regions has a thickness between 500 and 2000 angstroms. In anembodiment, the silicon germanium 301 deposition extends above thesurface of substrate 100. In this way the silicon germanium 301 isformed both above and below the surface of substrate 100. By formingsilicon germanium 301 above the surface of substrate 100, a raised tipis formed, increasing the conductivity of the tip. The increasedconductivity in turn improves device performance.

The silicon germanium 301 can further be doped to adjust its electricaland chemical properties. The doping can occur using a variety of dopantsand with a variety of doping techniques. For example, the silicongermanium 301 can be in situ doped with p-type impurities to a dopantconcentration level between 1*10¹⁸/cm³ and 3*10²¹/cm³ with aconcentration of approximately 1*10²⁰ cm³ being preferred. In anembodiment, and creating a PMOS device, the silicon germanium 301 isdoped with boron in situ during epitaxy by utilizing the precursorsnoted above and an additional B₂H₆ precursor gas as the source of theboron dopant during the silicon germanium 301 epitaxial deposition. Thebenefit of doping silicon germanium 301 in situ is that the undercutnature of undercut etch 201 makes it very difficult to dope the silicongermanium 301 after it has been deposited in area shadowed by thesidewall spacer. An angled implant, as one possible solution to dopingthe silicon germanium shadowed by the sidewall spacer, reduces the shortchannel performance of the resulting PMOS device.

In an embodiment, a fraction of the boron dopant added during thesilicon germanium 301 deposition is not activated at this time. That is,after deposition boron atoms are in the silicon germanium 301 layer buthave not yet substituted into silicon sites in the lattice where theycan provide a hole (i.e., a lack of an electron). In an embodiment, thethermal activation of the dopant is deferred until subsequent processingsteps, reducing the thermal budget and resulting dopant diffusion toenable a very abrupt source/drain junction to be formed, improvingdevice performance.

As introduced, the deposited silicon germanium 301 has a larger latticeconstant, the magnitude of which depends on the atomic percent germaniumin the silicon germanium 301 alloy. When deposited on the substrate 100silicon, the lattice of the silicon germanium 301 is compresses toaccommodate crystalline growth. The compression in the silicon germanium301 source and drain regions further creates compression in thesubstrate 100 region located between the silicon germanium 301 sourceand drain regions and beneath the insulator 104 region (i.e., thechannel of the MOS device). The compression creates an anisotropicatomic structure in the channel region, altering the conduction andvalence bands of the channel material. The compressive stress furtherreduces the hole effective mass in the channel area of substrate 100, inturn increasing hole mobility. The increased hole mobility increases thesaturation channel current of the resulting MOS transistor, therebyimproving the device performance.

FIGS. 4 a, 4 b, and 4 c illustrate the substrate cross section of FIG. 3during the creation of a silicide layer. More specifically, the layer isa self-aligned silicide or salicide layer. One skilled in the art willrecognize that silicide layer 402 is formed by depositing a thin layerof refractory metal, in an embodiment with standard sputteringtechniques (i.e., physical vapor deposition or “PVD”), on the silicongermanium 301 followed by subsequent processing steps to create themetal, silicon, and germanium silicide alloy. Silicide 403 differs inthat the semiconductor element of the silicide alloy depends on thematerial composition of gate 102.

Refractory metals include, among others, cobalt, titanium and nickel. Inan embodiment, the refractory metal is nickel. The selection of arefractory metal requires consideration of not only electricalcompatibility, but also mechanical and chemical compatibility with theunderlying silicon germanium 301 material occupying the undercut etch201 source and drain regions and the exposed source, drain, and gateregions of the corresponding NMOS devices on the same substrate. Forexample, the silicide layer 402 must be continuous and uniform to aidreducing interface resistance between the silicide layer 402 and theunderlying silicon germanium 301. Nickel tends to react uniformly withboth silicon and germanium, forming a stable ternary Ni(SiGe) phasewhereas cobalt and titanium react preferentially with silicon andsegregate the germanium component of the silicon germanium 301 alloy.Further, the titanium and cobalt based silicon germanium silicides havereduced thermal stability compared to nickel silicon germanium silicide.Improper refractory metal selection creates a non-ideal interfacebetween the silicide and semiconductor that increases the interfaceresistance independent of otherwise electrically compatible materials.

FIG. 4 a illustrates the substrate of FIG. 3 following the blanketdeposition of refractory metal 401. As noted, in an embodiment therefractory metal is PVD nickel. Environmentally, the PVD nickeldeposition occurs between 20° C. and 200° C. and at a pressure less than50 millitorr. The thickness of the nickel is between 50 and 200angstroms. The nickel deposition is followed by a rapid formation annealat between 325° C. and 450° C. for less than or equal to 60 secondsusing, for example, rapid thermal anneal (“RTA”) equipment. During theformation anneal the refractory metal 401 atop the silicon germanium 301and gate 102 reacts to form silicide 402 and silicide 403 respectivelyas illustrated by FIG. 4 b. As the nickel is deposited over the entireexposed surface of the substrate 100, the unreacted nickel (i.e., thenickel that has not reacted with silicon or silicon germanium to form asilicide with its underlying layer as it is deposited atop the sidewallspacer 103 nitride or the isolation 101 regions) is removed using a wetetch chemistry of, for example, a mixture of hot H₂O₂ and hot H₂SO₄. Theremaining reacted nickel atop the silicon germanium 301 source and drainregions and the gate 102 regions then undergoes a final anneal between400° C. and 550° C. to complete the nickel silicon germanium silicide402 and silicide 403 formation as illustrated by FIG. 4 c. The silicidelayer(s) may be further capped with, for example, a titanium nitride capto prevent the nickel silicon germanium silicide layer 402 and silicidelayer 403 from oxidizing during subsequent processing steps as is wellknown in the art.

FIGS. 5 through 7 illustrate the physics of how the proper materialselection for both the refractory metal and the source-drain regionmaterial reduce the corresponding contact resistivity. FIG. 5illustrates energy band diagrams for the bulk metal, p-type silicon, andsilicon germanium. The Fermi energy of the metal is denoted E_(Fm). TheFermi energies of the p-type silicon and silicon germanium are E_(FSi)and E_(FSiGe) respectively. As shown, though not necessarily to scale,the conduction band edge of the silicon germanium E_(CSiGE) is slightlylower than the conduction band edge of silicon E_(Csi). Further, thevalence band edge of the silicon germanium E_(VSiGe) is higher than thevalence band edge of the silicon E_(Vsi), and is proportionately higherbased on the percentage composition of germanium in the silicongermanium alloy. Accordingly, the energy band gap of the silicongermanium alloy is smaller than the energy band gap of silicon, theextent to which is, as introduced with reference to equations (1) and(2), dependant upon the percentage composition of germanium in thegermanium alloy.

FIG. 6 illustrates the band bending associated with contact between therefractory metal and the p-type silicon. Of note is the magnitude of theenergy barrier. With the p-type semiconductor, aligning the Fermi levelsat equilibrium mandates a positive charge on the metal side and anegative charge on the semiconductor side. The semiconductoraccommodates the negative charge by generating a depletion region inwhich ionized acceptors are left uncompensated by holes.

FIG. 7 illustrates the energy band bending associated with contactbetween the refractory metal and the silicon germanium 301 alloy. Againof note is the magnitude of the energy barrier, in this case therelative difference in energy barrier height of the silicon germaniumalloy when compared to p-type silicon. In other words, themetal-semiconductor work function is smaller for the metal-silicongermanium 301 contact versus the metal-p-type silicon contact. Currentconduction for the contact is dominated by tunneling. The specificcontact resistivity relationship is as follows:

$\begin{matrix}{\rho_{c} \propto {\mathbb{e}}^{\frac{4\;\pi\;\Phi_{B}}{qh}\sqrt{\frac{m^{*}ɛ}{N_{surf}}}}} & (3)\end{matrix}$The salient variables in equation (3) are the metal-semiconductor workfunction Φ_(B), the semiconductor doping N_(surf), and the effectivecarrier mass in the semiconductor m*. As noted, the effective mass forholes in a silicon germanium film is 0.34 m₀ versus 0.37 m₀ for silicon,where m₀ represents the rest mass of the electron.

The specific contact resistivity equation (3) illustrates that thespecific contact resistivity for a metal-semiconductor interface dependsprimarily on the metal-semiconductor work function, the doping densityin the semiconductor, and the effective mass of the carrier. Alteringany one of the variables, or combinations thereof, impacts the specificcontact resistivity. Utilizing silicon germanium 301, as noted,decreases the metal semiconductor work function and decreases thecarrier effective mass. In an embodiment, the silicon germanium isfurther doped as noted with reference to FIG. 3.

The interface between the silicon germanium 301 and the silicide 402 canbe further discussed as a metal-semiconductor ohmic contact. To begin,the effect of the energy barrier at the contact between the silicide andthe semiconductor must be viewed from a quantum mechanical view. As iswell know in the art, the wave-particle duality dictates that anelectron be treated as both a particle and a wave to determine how itbehaves. The energy barrier created by the silicide-semiconductorinterface can be viewed potential barrier of finite thickness andheight. For a given barrier height greater than the energy of anincident electron and a given barrier width, there is a certainprobability that the electron will penetrate the barrier and appear onthe other side. Such a tunneling phenomena is an important mechanism inthe conduction of electrons in solids.

More specifically, contact resistivity is a measure of how easilycurrent can flow across a metal-semiconductor interface. If a contact isohmic, by definition there is an unimpeded transfer of majority carriersfrom one material to another—in this case between the silicide 402 andthe silicon germanium 301. This can also be represented as a linearcurrent-voltage characteristic. In the case of a metal-semiconductorinterface, the conduction mechanism is dictated in part by the width ofthe semiconductor depletion region adjacent to the contact interface. Ifthe semiconductor is lightly doped (e.g., the Fermi energy is neitherclose to the conduction band edge energy or the valence band edgeenergy), the depletion region becomes wide enough that the only way foran electron to transfer between the two contact materials is by jumpingthe potential barrier by thermionic emission over the barrier maximum.Alternatively, if the semiconductor is heavily doped (the Fermi energyapproaching the conduction band edge energy for n-type and approachingthe valence band edge energy for p-type) the depletion region becomessufficiently narrow that the field emission, or carrier tunneling, isthe dominant conduction mechanism. Both field emission and thermionicemission contribute to conduction across the interface and can beengineered by, for example, material selection and doping.

FIG. 8 illustrates an embodiment of the invention utilizing silicongermanium 301 source and drain regions with, for example, a nickelsilicon germanium silicide layer 402. An external resistance R_(ext) 801is the total series resistance between the source (or drain) contact andthe channel of the intrinsic transistor. The interface resistancebetween the silicon germanium 301 and silicide 402 is a significantcomponent of the total series resistance. R_(ext) 801 has been reducedby using silicon germanium versus using p-type silicon for the sourceand drain regions. Further, proper selection of the silicide layer 402refractory metal, in an embodiment nickel, has ensured a chemically andmechanically compatible metal-semiconductor interface that does notadversely contribute to R_(ext) 801 as noted with reference to FIG. 4.

One skilled in the art will recognize the elegance of the disclosedembodiment in that it decreases the specific contact resistivity betweenthe silicon germanium source and drain regions and their respectivesilicide contacts by utilizing novel material selection and processingtechniques. As the contact resistivity contributes to the overallexternal resistivity of the transistor, a reduction in the contactresistivity contributes to an overall performance increase in thetransistor.

What is claimed is:
 1. A method comprising: forming an insulator regionon a substrate having a first lattice constant; etching a source regionand a drain region in the substrate, thereby defining a channel regionlocated between the source region and the drain region, and beneath theinsulator region; depositing a semiconductor material in the sourceregion and in the drain region, the semiconductor material having asecond lattice constant larger than the first lattice constant; whereinthe semiconductor material in the source region and in the drain regioncreates compression in the channel region; and forming contacts on thetop surface of the semiconductor material in the source region and inthe drain region; wherein the contacts comprise an alloy of nickel andthe semiconductor material.
 2. The method of claim 1 wherein thesemiconductor material is a silicon germanium alloy.
 3. The method ofclaim 2 wherein the contacts comprise nickel silicon germanium silicide.4. The method of claim 2 wherein the silicon germanium alloy has agermanium composition between 5% and 50%.
 5. The method of claim 4wherein the silicon germanium alloy has a germanium composition between10% and 40%.
 6. The method of claim 5 wherein the silicon germaniumalloy has a germanium composition between 15% and 30%.